1. Field of the Invention
The invention relates to a method and an apparatus for setting the imaging properties of an optical system with radiation treatment of at least one internal optical element of the optical system in the installed state, and to a method for setting the imaging properties of an internal optical element with radiation treatment.
2. Description of Related Art
U.S. Pat. No. 6,255,619 B1 describes a method and an apparatus for material-removing radiation treatment of an exit surface of a semiconductor laser element, or of a light exit lens of a semiconductor laser element, or of another light source, in the case of which the shape of the wavefront of radiation that exits from the exit surface to be treated is detected by a wavefront measuring apparatus. A control unit infers the shape profile of the treated exit surface from the measured shape of the wavefront and compares the actual shape profile thus determined with a desired shape profile that can be prescribed in order to achieve desired imaging properties, and drives a pulsed UV processing laser as a function thereof. The processing radiation delivered by said laser is directed by a beam splitter mirror onto the exit surface in order to produce the desired shape profile there by material removal, and thus to obtain the desired imaging properties. The control unit can set the position of the beam splitter mirror such that the processing radiation can be directed onto respectively desired surface regions of the light exit surface to be treated, and in the reverse direction passes the radiation emerging from the treated exit surface to the wavefront measuring apparatus.
An important application of the invention is imaging systems for microlithography, which must have a high imaging quality in order to fulfill the requirements placed on them. It is frequently expedient in this case also to make use of aspheric surfaces or elements, briefly denoted as aspherics. For a projection objective of a lithography machine, it is known from U.S. Pat. No. 6,268,903 B1 to arrange an optical distortion correction element upstream of the objective, the correction element firstly being placed upstream of the objective and a distortion measurement being carried out in order subsequently to calculate therefrom a correcting, aspheric desired shape of the surface for the correction element, to remove the correction element from the beam path and process it correspondingly by removing material, and then to position it again upstream of the objective.
For a projection exposure machine, it is proposed in the laid-open specification EP 0 823 662 A2 to irradiate a correction radiation during an exposure operation in addition to the exposure radiation, doing so in such a way as to suppress and/or compensate as far as possible a nonuniform temperature distribution, caused by the exposure radiation, of optical elements of a projection objective and a variation, associated therewith, in the imaging properties of the respective optical element owing to the influence of the correction radiation. Further proposed in the laid-open specification DE 101 40 208 A1 is a correction radiation device with the aid of which such a correction radiation can be guided in a scanning fashion over a defined surface region of the relevant optical element.
U.S. Pat. No. 6,205,818 B1 discloses a method for precompacting optical elements made from fused quartz in order to protect against damage by laser radiation a region of the optical element used during operation, that is to say transirradiated. This precompacting can comprise, for example, an irradiation with pulsed laser radiation having a higher pulse flux and/or shorter wavelength than during operation, an irradiation with a high-energy electron beam, or a hot isostatic pressing in order, for example, to raise the density of the optical element by 10 ppm to 3%.
Laid-open specification US 2002/0060781 A1 describes a method for aberration correction of an optical system that makes use of an aberration compensation layer that can be set by irradiation and is attached to an optical element of the optical system. Before or after attaching this compensation layer, the aberration to be corrected is determined in order then as a function thereof to irradiate the compensation layer for the purpose of changing its refractive index, doing so in such a way that the aberration is compensated. Such compensation layers typically comprise special compositions of a polymer matrix with a polymerizable compound that modulates the refractive index. Before, during and/or after the radiation treatment of the compensation layer, the aberrations can be measured by means of a diagnostic system that includes an interferometric wavefront measurement apparatus, for example of the Shack-Hartmann type. After the aberration measurement, a typical method cycle includes the determination of the required intensity profile for the compensation layer for the correction of the measured aberrations, the placing of the required intensity profile on a static mask or a programmable mask generator, a control mode of a calibration camera for the correction of the mask with the aim of compensating aberrations in a projection optics and nonuniformities of a light source, the irradiation of the compensation layer for a prescribed time period with the aid of a suitable wavelength, intensity and spatial distribution of the radiation, and a renewed measurement of the aberrations of the optical system after a certain waiting time. If necessary, this method cycle can be repeated until success of the correction lies within desired limits.